In general, a compression refrigerator includes at least a compressor, a condenser, an expansion mechanism (such as an expansion valve), and an evaporator, or further includes a drier, and is structured so that a mixed liquid of a refrigerant and lubricant (refrigerator oil) circulates in a closed system. In such a compression refrigerator, the temperature in the compressor is generally high, and the temperature in the condenser is generally low, though such general theory depends on the kind of such a compression refrigerator. Accordingly, the refrigerant and the lubricant must circulate in the system without undergoing phase separation in a wide temperature range from low temperature to high temperature. In general, the refrigerant and the lubricant have regions where they undergo phase separation at low temperatures and high temperatures. Moreover, the highest temperature of the region where the refrigerant and the lubricant undergo phase separation at low temperatures is preferably −10° C. or lower, or particularly preferably −20° C. or lower. On the other hand, the lowest temperature of the region where the refrigerant and the lubricant undergo phase separation at high temperatures is preferably 10° C. or higher, more preferably 30° C. or higher, or particularly preferably 40° C. or higher. The occurrence of the phase separation during the operation of the refrigerator adversely affects the lifetime and efficiency of the refrigerator to a remarkable extent. For example, when the phase separation of the refrigerant and the lubricant occurs in the compressor portion, a movable part is insufficiently lubricated, with the result that baking or the like occurs to shorten the lifetime of the refrigerator remarkably. On the other hand, when the phase separation occurs in the evaporator, the lubricant having a high viscosity is present, with the result that the efficiency of heat exchange reduces.
A chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), or the like has been heretofore mainly used as a refrigerant for a refrigerator. However, such compounds each contain chlorine that is responsible for environmental issues, so investigation has been conducted on a chlorine-free alternative refrigerant such as a hydrofluorocarbon (HFC). A hydrofluorocarbon typified by, for example, 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, or 1,1,1-trifluoroethane has been attracting attention, and, for example, 1,1,1,2-tetrafluoroethane has been used in an air-conditioning system for cars
However, because the influence of the HFC is also concerned from the viewpoint of the global warming, so-called natural refrigerants such as carbon dioxide have attracted attention as alternative refrigerants more suitable for environmental protection. The carbon dioxide requires high pressure, and hence cannot be used in the current air-conditioning system for cars.
A refrigerant having a specific polar structure in the molecule such as an unsaturated fluorinated hydrocarbon compound (see, for example, Patent Document 1), a fluorinated ether compound (see, for example, Patent Document 2), a fluorinated alcohol compound, or a fluorinated ketone compound has been found as a refrigerant which has a low global warming potential and can be used in a current air-conditioning system for cars.
The lubricant for a refrigerator that uses the refrigerant is demanded to have excellent compatibility with the refrigerant and excellent stability.    Patent Document 1: JP 2006-503961 A    Patent Document 2: JP 07-507342 A